Photovoltaic (PV) panels (here also referred to as “solar panels”) use radiant light from the sun to produce electrical energy. The solar panels include a number of PV cells to convert the sunlight into the electrical energy. The majority of solar panels use wafer-based crystalline silicon cells or a thin-film cell based on cadmium telluride or silicon. Crystalline silicon, which is commonly used in the wafer form in PV cells, is derived from silicon, a commonly used semi-conductor. PV cells are semiconductor devices that convert light directly into energy. When light shines on a PV cell, a voltage develops across the cell, and a current flows through the cell when connected to a load. The voltage and current vary with several factors, including the physical size of the cell, the amount of light shining on the cell, the temperature of the cell, and external factors.
A solar panel (also referred to as a “PV module”) is made of PV cells arranged in series and in parallel. For example, the PV cells are first coupled in series within a group. Then, a number of the groups are coupled together in parallel. Likewise, a PV array (also referred to as a “solar array”) is made of solar panels arranged in series and in parallel.
The electrical power generated by each solar panel is determined by the solar panel's voltage and current. In a solar array, electrical connections are made in series to achieve a desired output string voltage and/or in parallel to provide a desired amount of string current source capability. In some cases, each panel voltage is boosted or bucked with a DC-DC converter.
The solar array is connected to an electrical load, an electrical grid or an electrical power storage device, such as (but not limited to) battery cells. The solar panels deliver direct current (DC) electrical power. When the electrical load, electrical grid or electrical power storage device operates using an alternating current (AC) (for example, sixty cycles per second or 60 Hz), the solar array is connected to the electrical load, electrical grid, or electrical power storage device, through a DC-AC inverter.
Solar panels exhibit voltage and current characteristics described by their I-V curve, an example of which is shown in FIG. 1. When the solar cells are not connected to a load, the voltage across their terminals is their open circuit voltage Voc. When the terminals are connected together to form a short circuit, a short circuit current Isc is generated. In both cases, since power is given by voltage multiplied by current, no power is generated. A Maximum Power Point (MPP) defines a point where the solar panels are operating at a maximum power.
Often, a solar panel is capable of large and fast power transients. During these transients, the difference between the power generated by the solar panel and the power put on the grid by the inverter (e.g., in the case of a solar array connected to the grid) is stored and released by an electrical energy storage device (e.g., an inverter input capacitor). Under certain conditions referred to as a string over-voltage, the power difference can cause the inverter input voltage to exceed the inverter's maximum rating, causing severe and permanent damage to the inverter.